fix(cascade): reconcile vanished source paths as deletes

[SiyaoZheng]

Summary

Reconcile stale add/modify cascade events as deletes when their source Markdown path has disappeared before processing.

The filesystem watcher can enqueue an add or modify event, then the path can be deleted or replaced before the worker reads it. The resulting FileNotFoundError was treated as a permanent failure, leaving an old row indexed even though the source of truth no longer existed.

The worker now checks the source path when a handler raises FileNotFoundError:

• if the source path is gone, it runs the handler's idempotent delete path;
• if the source still exists, it treats the error as recoverable so an unrelated missing dependency is not mistaken for source deletion.

Area

• Architecture method
• Benchmark
• Use case
• Documentation
• Developer experience
• CI, build, or release

Verification

uv run pytest tests/unit/test_memory/test_cascade/test_worker.py -q
24 passed

make ci
1501 unit tests passed; 78 integration tests passed (6 deselected);
lint, architecture contracts, OpenAPI drift, package build, and wheel smoke test passed

Checklist

• I kept the change scoped to the relevant area.
• I am opening this from a separate branch, not pushing directly to main.
• No documentation change is required because the public API and configuration are unchanged.
• I added or updated tests when the change affects behavior.
• I did not commit secrets, .env files, dependency folders, or generated output.
• Active relative links in Markdown files resolve.

Notes for Reviewers

The regression tests cover both the vanished-source reconciliation path and the guard that preserves retry behavior when the source still exists.

By submitting this pull request, I agree that my contribution is licensed under the Apache License 2.0.